fix problematic HTML caused by certain trailing backslashes

In cases that are **outside of a markdown code block**,
a markdown line with ending in a tailing "\" can generate
problematic HTML.

We certain trailing backslashes outside of a markdown code block
with "<br>" HTML elements, or in some cases eliminated the
need for a trailing backslash.

In a few cases some unnecessary trailing backslashes inside
of a markdown code block were removed.

The following markdown files were changed in this commit:

```
1990/dds/README.md
1990/jaw/README.md
1991/ant/README.md
1991/rince/README.md
1991/westley/README.md
1992/vern/README.md
1992/westley/README.md
1998/schweikh1/README.md
2004/anonymous/README.md
2005/sykes/README.md
2006/toledo2/README.md
2011/toledo/README.md
2012/endoh2/README.md
2012/tromp/README.md
2013/robison/README.md
2015/hou/README.md
2015/howe/README.md
2015/schweikhardt/README.md
2018/burton1/README.md
2018/burton2/README.md
2018/ferguson/README.md
2018/vokes/README.md
2020/ferguson1/README.md
```

Performed `make www` to test these changes.

2012/tromp/README.md

@@ -77,8 +77,11 @@ That's all there is to
 [BLC](https://tromp.github.io/cl/Binary_lambda_calculus.html)!
 
 For example the encoding of lambda term `S = \x \y \z  (x z) (y z)`,
-with [De Bruijn notation](https://en.wikipedia.org/wiki/De_Bruijn_notation) `\ \
-\ (3 1) (2 1)`, is `00 00 00 01 01 1110 10 01 110 10`.
+with [De Bruijn notation](https://en.wikipedia.org/wiki/De_Bruijn_notation):
+
+```
+    \ \ \ (3 1) (2 1)`, is `00 00 00 01 01 1110 10 01 110 10
+```
 
 In the closely related
 [BLC8](https://tromp.github.io/cl/Binary_lambda_calculus.html#BLC8:_byte_sized_I.2FO)